The three items in this FAQ are all interacting, each in their own specific way and yet, where the amateur is concerned, All three are working either for or against each other.

Cross section is a variable totally dependent on applied voltage. The "fusion cross section" is a probabilty that in a uniformly active and constant velocity flux of fusion fuel, (deuterons for us), that fusion will take place. Given this definition, the probability of fusion goes up the faster the average fusion fuel item, (deuteron), is moving, (Kinetic energy). In a plasma or accelerated stream of deuterons, a higher voltage accelerates the deuterons to a higher velocity, (higher kinetic energy). Thus, the more voltage applied, the faster the deuterons move and the higher the cross section, (probability), of fusion taking place.

Is there a minimum voltage to do fusion? Is there an ideal voltage? Is there a maximum voltage? An obvious question for the would-be amateur fusioneer.

The real, yet complex, answer follows.

Theoretically, there is literally no effective lower limit of a real significant applied voltage where fusion is zero. Fusion can and does occur at neon sign transformer voltages, (3-12kv).
Theoretically, there is a practical peak output applied fusion voltage based on a maximized return of fusion per watt input to the plasma. This occurs just short of 200kev
Theoretically, there is a maximum voltage where no more increase in fusion cross section and, thus fusion will occur. The maximum applied voltage would be about 3mev

Leaving theory behind.....We are talking hands-on amateur fusion in the here and now. The amateur is typically young, poor, excited and wanting to get busy and do fusion. As we note rather often "fusion is easy" with a proviso that you have the funds and pack the gear and verve needed to see it through...........amateur fusion can be done with a capital outlay of between $1000.00 and $25,000 based on many factors covered in a number of other FAQs related to costs to be found in the construction forum FAQs.

The key road blocks to doing and proving fusion is not the reactor assembly, the vacuum system or the deuterium gas supply, themselves, (though they are important enough)! Three key items that most consider after thoughts and relate to this discussion's three main tenats are listed as follows:

Yes, fusion can and does occur at 10kv, but you will never detect it. Why? You would have to spend on the order of $3000 or more on a plus-ultra neutron detection scheme and do a lot of controlled statistics to show you are doing fusion. A dead zone tends to exist in amateur fusion up to 15kv and often to 20kv where the average amateur fusioneer will not be properly equipped to detect fusion that is taking place, (failure to confirm fusion).

The general threshold for moderately easy detection of fusion begins in the range of 25-30kv applied. However, it gets much easier above 30kv applied.

It is important to remember that more voltage means more fusion and more fusion is more easily detected and proven to have taken place.

Is there a point,(in amateur fusion), where more voltage becomes a bit less productive based on the horrors of having to standoff issues associated with such high voltages?

YES! Studying very carefully the fusion cross section curves for d-d fusion, one notes an order of magnitude improvement in fusion between 20kv and about 38kv...Nice!.... The next order of magnitude improvemnt occurs over the range of 38kv to 120kv....Ouch! An indepth study of the curves between 38 kv and 120kv shows a nice usable zone of significant improvement up to about 60-70kv applied where gains are still good related to having to deal with the increased voltage handling issues. Based on the zealous, well funded amateur, I would think that 70kv is the absolute limit for amateur fusion in a fusor. X-rays produced at this 70kv level are fearsome and dangerous! From personal experience, 45 kv applied is easy to work with and will give a good fusion return capable of doing many activation experiments.

An ideal amateur fusion, research capable, voltage might be fixed at 40kv (ease of containing and handling the high voltage, ease of detection, good future fusion experimental capability)

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CURRENT:

Herein, comes the rub in amateur fusion. High voltage supplies abound and are really very inexpensive!!! Most such supplies are positive hot and negative ground. These are totally un-usable for fusion in a fusor. Some rare supplies are offered that are negative hot and positive grounded, but are typically very expensive. Some few supplies allow reversal internally between postive or negative hot. However, virtually 100% of all offered supplies, regardless of type, will never have sufficient current capability needed to do fusion!!!

So many amatuers purchase a great supply, voltage wise, only to find out it will not carry the water to do fusion due to a limited current capability. The amteur must seek out a supply that is negative hot and positive grounded of a voltage that is capable of doing easily detectable fusion and a current sufficient to create enough deuteron flux to support the voltage in this effort.

At any fixed voltage, be it 10kv or 50kv, applying more current will create more current carriers, (deuterons). More deuterons moving at fusion energy means more fusion! Too much current will heat and ultimately melt the wire grid in a fusor. It has been found over the last 20 years, in the amateur fusion arena, that 10 milliamperes, (10ma), at fusion voltage does good fusion when running. However a reserve demand of 20-30 ma capability in a supply is a must have!!! This current demand puts extreme limits on what is to be found in the way of a useful fusor supply in the market place for under $5000.00. Wow! There is that money thing again. Many amateurs will wind up having to construct a suitable supply using surplus x-ray transformers, variacs, high voltage diodes and metering systems. This may be done relatively inexpensively, but will require significant electrical or electronics skills. High voltage safety becomes of paramount importance to avoid being killed or injured when working with fusion supply voltages and currents.

Supply summary for amateur fusion..........

To sum up, the ideal fusor has a fully adjustable power supply of a minimum of 0-30kv and 0 to 20-30 ma capabilty and presents its output as negative hot and positive grounded. A serious supply whether purchased or hand-built must have metering of both the voltage and current output!

This subject is dealt with extensively in the FAQs on same in the neutron and radiation detection forum. Suffice it to say here, that there are many methods of detection and, thus, proving fusion is taking place. It is a matter of balancing you power supply capabilities against your detection scheme's sensitivity to do, and then, detect fusion. This is another big rub and stumbling block in the amateur fusion quest. Big money and complex detection schemes are needed to detect really weak fusion in the 20kv range. If you can work in the 30kv+ range with a 20ma supply, silver activation and a simple geiger counter can prove fusion. Read up on the various methods in the above mentioned FAQs.

Remember, detection goes hand-in-hand with the fusioneer's power supply. If you are serious about amateur fusion, look at the supply issues and the detection issues first before you go off building a fusion reactor chamber and vacuum system.

Voltage determines cross section and fusion probability; more voltage means a higher probabilty that fusion will occur at all.
Current determines the flux of the fusion fuel in the fusion reaction. More current at any voltage means more fusion.
The above criterion of the supply, its capabilities or lack thereof, will determine the detection method that must be brought to bear to prove fusion is even taking place.

Richard Hull

Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Outstanding post, Richard. Clear, concise and sums up all the critical issues that face anyone desiring to construct a real fusor. This should be the mandatory first FAQ read by any and all people here who say they want to build a fusor. You have provided good physics explanations for the main details of a fusor, and the explanations logically provide all the details that are critical for any novice before they attempt to start on this difficult, though rewarding, enterprise.

I'm trying to build a fusion reactor and I was wondering what kind of voltage source would work. Can I just make a current rectifier and a boost converter and then plug it into the wall? Is a standard AC wall outlet neutral hot all the time, none of the time, or half the time? Would the voltage output from a boost converter be stable enough for the reactor to function properly, or do I need find a flyback converter?

In theory, yes, and that is how the beautiful expensive Spellman and Glassman HV power supplies work.

However, designing a switching power supply that can provide the extreme voltage needed at a sufficient current is very challenging. Doing so without killing yourself or blowing up your oscilloscope is harder. It is very much a "if you have to ask, you can't do it" type thing. Electricity behaves very strangely above 30 kV.

These supplies usually use a frequency of some tens or hundreds of kHz to drive a transformer stepping voltage up to maybe 5 to 12 kV. Then a voltage multiplier multiplies it to the desired voltage while rectifying it. Feedback for such a high voltage boost converter is difficult.

As to the humble wall plug, the only answer I can give is that it depends what country you are in and whether the electrician wired your house correctly. The only safe choice is to assume nothing. I'm assuming you mean "neutral", not negative. It ought to be at ground all the time, but many people have been shocked or killed by trusting body to that assumption.

On the original subject it is important to note that most of the current in a fusor without ion guns is wasted by passing thru the glow discharge, heating the walls and the grid. This is necessary to keep the glow discharge running which is a competing requirement with having fusion happen.

A Beam-On-Target reactor can operate on a fraction of a milliampere under much higher vacuum however it is more complex to build than a fusor and needs at least two and possibly three power supplies (only one of which must be fusor spec). It does this because the only current flowing in such a reactor is in the beam of deuterons.